This commit introduces a new Sign In / Sign Up form for trials.
It behaves almost like the original with some exceptions:
- removed ability to sign in via social networks;
- removed captcha on the sign in form;
- field 'name' replaced by two new fields 'first_name' and 'last_name';

This removes the `dependency_list_vulnerabilities` feature flag and all
code/template/spec branches associated with the missing/disabled feature
flag case.
This completes the feature[1] for adding dependency scanning results to
the Dependency List.
[1]: https://gitlab.com/gitlab-org/gitlab-ee/issues/10077